Directed Flow at RHIC Based on Transverse Deflection of Spectator Neutrons

Directed Flow at RHIC Based on Transverse Deflection of Spectator

NeutronsGang Wang (UCLA)

for STAR Collaboration

Introduction to directed flow

Detectors: ZDC-SMD, (F)TPC

Method and check

Comparison between 3 systems

Summary

x

yz

Gang Wang (UCLA) Jun 7, 2006 2

r

pt

Higher harmonics

rapidity

<px> or directed flow

)](cos[

)](cos[212

1

1

2

3

3

rn

nrn

tt

nv

nvdydpp

Nd

pd

NdE

Anisotropic flow

Directed flow (v1)

Elliptic flow (v2)


v1(y) structureModels predict that anti-flow/3rd flow component, with QGP v1(y) flat or a wiggle structure.

L.P. Csernai, D. RohrichPLB 458, 454 (1999)

J. Brachmann et al.PRC 61, 024909 (2000)

Hydro-dynamics modelA . Nyiri, L.P. Csernai, et al., J. Phys. G31, S1045 (2005)


v1(y) structureModels without QGP also predict a wiggle structure in peripheral or mid-peripheral events.

M. Bleicher and H. Stöcker, PLB 526, 309 (2002)

RQMD

UrQMD

R. Snellings, H. Sorge, S. Voloshin, F. Wang, N. Xu, PRL 84, 2803 (2000); also H. Liu et al., PRC 59, 348 (1999).

Baryon stopping and positive space-momentum correlation


RHIC

Easier to measure

Harder!

Non-flow effects

Flow fluctuations

Low statistics


STAR Main Detector


STAR ZDCEach of the RHIC experiments has a pair of Zero Degree Calorimeters for beam monitoring, triggering, and locating interaction vertices.

ZDCs detect neutrons emitted along beam directions and measure their total energy (multiplicity) .

Baseline ZDCs have no transverse segmentation, which motivates upgrade.


STAR ZDC-SMD

SMD is 8 horizontal slats & 7 vertical slats located at 1/3 of the depth of the ZDC

• New knowledge of the direction of the impact parameter vector• Minimal, if any, non-flow effects• Minimal, if any, effects from flow fluctuations• Worse resolution than from TPC, but that disadvantage is minor

ZDC side view

Scintillator slats of Shower Max Detector

Transverse plane of

ZDC


Flow study with ZDC-SMD: 4 terms

In analysis:

For example:

To systematically study the method with ZDC-SMD, we can use the sub event plane from only east or west ZDC-SMD, instead of the full event plane, and even break down the correlation into X and Y direction.

Definitions of 4 terms:


First result: cross check

Au +Au 62 GeV

Models do well at larger || but their v1() is too flat near = 0

STAR Collaboration, Phys. Rev. C 73, 034903 (2006)


Comparisons between 3 collision

systems: 62 GeV Au +Au200 GeV Au +Au200 GeV Cu +Cu


Au +Au 62 GeV10% - 70%

Mid-rapidity

Charged particle v1 is a monotonic function of η.

Au +Au 200 GeV

AMPT model gives a


Forward rapidity

Au +Au 62 GeV

AMPT model gives a

AMPT model shows a similar tendency, with smaller magnitude.

AMPT model gives a

Hydro-dynamics modelA . Nyiri, L.P. Csernai, et al., J. Phys. G31, S1045 (2005)


More Hydro?Au +Au 200 GeV

Hydro predicts the sign change in v1(pt)U. Heinz and P. Kolb, J. Phys. G30, S1229 (2004)

p-bar flows with proton

Assume (anti-)proton and pion v1(pt) are straight lines starting from zero, and neglect other particles. Charged particle v1(pt) can be fit simply.


Scaling behavior

The hypothesis holds here.

It has been observed that particle emission(both spectra and flow) as a function of rapidity in the vicinity of beam rapidity appears unchanged over a wide range of beam energies, a pattern known as limiting fragmentation.


10% - 60%

More Scaling

STAR

preliminar

y


Summary• For all 3 collision systems, charged particle v1(η) decreases with η in

mid-rapitidies, flowing opposite to spectators.

• Charged particle v1(η) approaches zero at forward η sooner or later. The more central collision, the sooner.

– AMPT model predicts this effect, with much smaller magnitude. (cross-section?)– Hydro-dynamics model also predicts the zero-approaching.

• Charged particle v1(pt) crosses zero at pt > 1 GeV/c in the TPC region at 200 GeV Au +Au. The more central collision, the sooner.

– Hydro-dynamics model predicts the sign-change.

– With an assumption of the straight-line shape in v1(pt) for identified particles, the slopes can be fit with particle yields.

– Identified particle v1 still need more investigation.

• Limiting fragmentation is observed when comparing 62 GeV and 200 GeV Au +Au.

• System size doesn’t matter in v1? – 200 GeV Au +Au and Cu +Cu– Would look at 62 GeV Cu +Cu, and compare it with 62 GeV Au +Au.

• v1 results call for more input from model calculations.


Backup Slides


Identified particles : v1 in 62 GeV Au +Au

STAR Collaboration, Phys. Rev. C 73, 034903 (2006)

Documents

Directed Flow at RHIC Based on Transverse Deflection of Spectator Neutrons